Abiotic and biotic factorsSpec 4.5
In short
Abiotic factors are non-living factors, such as temperature, light and water, and biotic factors are living factors, such as competition and predation. Both affect population size and distribution. Organisms compete for limited resources like food, water, light and space, and predator and prey numbers rise and fall in a cycle, with the predator peak coming after the prey peak.
The population size and distribution of organisms are affected by abiotic factors (non-living) and biotic factors (living). Changes in either can change which species live in a place and how many there are.
| Type | Factor | How it can affect population size and distribution |
|---|---|---|
| Abiotic | Temperature | Affects the rate of enzyme-controlled reactions, so growth and survival. Species survive only within a range of temperatures. |
| Abiotic | Light | Plants need light for photosynthesis. Less light means fewer plants, and so less food for the animals that depend on them. |
| Abiotic | Water | All organisms need water. In dry conditions only organisms adapted to save water survive, so numbers and variety fall. |
| Biotic | Competition | Organisms of the same or different species compete for the same limited resources, such as food, water, light and space. |
| Biotic | Predation | Predators eat prey, so predator and prey numbers affect each other. |
Competition
Competition happens when organisms need the same resource and there is not enough for all of them. Plants compete for light, water and mineral ions. Animals compete for food, water and territory. The better-adapted species gets more of the resource, so it survives and reproduces more, and the numbers of the other species may fall.
Predation
In a stable community, predator and prey numbers rise and fall in a repeating pattern. When prey numbers rise, predators have more food, so predator numbers rise. More predators eat more prey, so prey numbers fall. With less food, predator numbers then fall, and the prey can recover.
On a predator-prey graph, the predator peak comes after the prey peak, because predator numbers rise only once there is more food.
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What is a population?
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All the organisms of one species living in the same area.
What is an ecosystem?
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A community together with the non-living (abiotic) parts of its environment.
Why are quadrats placed randomly?
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To avoid bias.
Biology only What is biodiversity?
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The variety of different species in an ecosystem or habitat.
Name two biotic factors that affect population size.
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Competition and predation.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026